Effect of surface roughness on breakdown behavior of Al-Zn-Mg-Cu alloy

The surface microstructure and corrosion breakdown behavior of AA7055 Al alloy with different surface roughness were studied using scanning transmission electron microscope and polarization measurements. Atom probe tomography was used to study the compositions of the solid solution matrix of AA7055 alloy in three typical tempers. The matrix Cu content decreased from peak-aged to over-aged tempers, which resulted in a reduction in pitting potential. The AA7055-T77 sample abraded to 320 grit exhibited two breakdown potentials, whereas the 400 grit abraded sample exhibited only one. Surface layer attack occurred for both samples, but was absent for the 1200 grit abraded sample, which exhibited one breakdown potential. Abrasion-induced surface layers containing various microstructural features were observed on all surfaces. Rougher surfaces exhibited thicker surface layers with more Znenriched sub-boundaries and fewer ρ′ precipitates. These changes in surface microstructure with surface roughness resulted in difference in susceptibility of the surface layer, and in the activity gap between the surface layer and underlying bulk matrix, which altered the breakdown behavior of AA7055 alloy.